This invention relates to a honeycomb structure or matrix of the type having thin-walled, open-ended cells extending generally along the axis of the structure. More particularly, this invention relates to a method and apparatus for extruding such a structure.
A structure of this type may be made by advancing a batch of extrudable material such as a cordierite mixture toward an extrusion die having a plurality of feed holes supplying a gridwork of discharge slots. Typically, a backing plate having a generally cylindrical opening defining the desired cross-sectional shape or area of the structure which is substantially smaller than the cross-sectional area of the extrusion barrel itself is located immediately behind the die. The front of the die may be fitted with a suitable mask so as to crush the peripheral cells of the extruded structure thereby forming a substantially uniform skin around the periphery of the structure.
When the extrudable material reaches the backing plate, there is a necessity for the extrudable material to be directed radially inwardly toward the cylindrical opening in the backing plate. The flow, which may be achieved by the accumulation of a stagnant zone of extrudable material behind the backing plate radially outwardly from the opening, becomes a shearing flow of the extrudable material, i.e., flow having an inwardly radial flow and velocity gradient. If the peripheral or other portion of the extrudable material is in a state of radial shearing flow at the time the material enters the extrusion die, there is a resulting non-uniformity of flow at and through the extrusion die. The non-uniformity of flow can produce irregularities in the skin if the flow is too slow in the peripheral portions of the die vis-a-vis the flow through the central portions of the die. An example of such irregularities is the tearing of the skin especially where there is an interface between two separate batches or billets of extrusion material. In the alternative, the non-uniformity of flow can result in the crushing of cells near the periphery of the matrix adjacent the skin if the flow is too fast through the peripheral portions of the die. These problems are only compounded when the structure itself is noncircular in cross-section perpendicular to the direction of the flow, i.e., axially asymmetric, especially where the noncircular or asymmetric configuration has a high aspect ratio, e.g., the major to minor axis ratio of the noncircular cross-section is substantially greater than one. Structures having such a high aspect ratio include oval or "racetrack" designs which are commonly utilized in catalytic converters for vehicles where the structure comprises a ceramic material.
U.S. Pat. No. 3,888,963--Orso et al. discloses an extrusion apparatus for making a honeycomb structure wherein the extrusion die is preceded by a homogenizing assembly forming a straight-walled entrance cavity for the die having a cross-sectional area substantially corresponding with the cross-sectional area of openings in the homogenizing assembly. The cross-sectional area of the cavity substantially corresponds with the cross-sectional, flow-through area of the exposed inlet face of the extrusion die and thus the structure to be extruded thereby eliminating any shearing flow at the extrusion die. A similar straight-walled cavity immediately preceding the extrusion die is disclosed in U.S. Pat. No. 4,168,944--Morikawa et al.
Japanese Patent application No. 1977-51792, filed May 4, 1977 (Japanese Unexamined Patent Application Publication No. 53-137260 dated Nov. 30, 1978) discloses an extrusion die preceded by a cavity which expands outwardly from a smaller cross-sectional area remote from extrusion die to a larger cross-sectional area adjacent the extrusion die. This apparatus relies on the principle that a convex shearing profile may be flattened by passing the batch material through an expansion zone immediately preceding the die. However, the expansion zone becomes dimensionally critical, i.e., the cavity must have a certain length. Moreover, the rate of expansion itself becomes quite critical to assure flattening of the convex shearing profile. This criticality in expansion is also somewhat complicated when applied to structures of noncircular or axially asymmetric configuration. Furthermore, the outward taper or funnel of the Japanese patent application is critically dependent upon the rheology of the extrusion material. Finally, the outward taper or funnel of the Japanese application can result in stagnant zones immediately adjacent the extrusion die which can produce shearing flow at the die with adverse effects on skin quality or resultant cell crushing.